1. Field of the Invention
The present invention concerns a corrosion resistant resin coating structure in a metal tube, among all, a metal tube of a tube diameter, particularly, of less than 30 nm, having excellent adhesion and excellent chemical and mechanical corrosion resistance, which is disposed in plurality as pipelines for brake oil or fuel of automobiles or feed channels for feeding oil or air to various kinds of machines and equipments.
2. Description of the Prior Art
A protection coating has been applied to an outer surface of a metal tube. For instance, pipelines for brake oil or fuel of automobiles undergo severe working conditions since they are disposed to an under floor surface of the automobiles, and they are required to have chemical resistance, as well as scratch resistance, impact resistance or resistance to injuries caused by external mechanical forces. Further, corrosion of pipelines caused by rock salt as a road anti-freezing agent has presented a further problem. Accordingly, it has been desired for a metal tube structure applied with a protection coating capable of withstanding mechanical erosion and also chemical corrosion, for which various proposals have been made.
That is, there have been known protection coatings, for example, formed by applying a zinc plating layer by electric plating on an outer surface of a metal tube, forming a relatively thick olive-colored specific chromate film on the plating layer and forming a fluoro resin layer further thereon (for example, U.S. Pat. Nos. 3,808,057 and 4,003,760), forming a polyvinyl fluoride layer comprising two fluoro resin layers fused to each other (Japanese Utility Model Publication Hei 3-11517) and, further, forming an epoxy resin intermediate layer between a chromate film and a polyvinyl fluoride layer (Japanese Patent Application Sho 62-84429).
In such existent corrosion resistant resin coating structure in such metal tubes, an epoxy resin intermediate layer or a fluoro resin such as a polyvinyl fluoride resin is coated by way of a relatively thick chromate film formed from a chromate solution comprising hexavalent chromium ions on a zinc plating layer applied at least to an outer circumferential surface of a tube material. However, since the resin layers are baked at a high temperature, it results in a problem that the chromate film suffers from remarkable thermal degradation and the corrosion resistance inherent to the chromate film is inevitably reduced by the heat treatment. Further, formation of the chromate film with a chromate solution generally involves a problem that hexavalent chromium ions contained in a treating solution and an acid added as a reducing agent are consumed in a great amount, so that each of ingredients of the treating solution has to be supplemented and renewed at a predetermined interval for maintaining a certain layer-forming performance, which increases a cost for disposing wastes containing a great amount of deleterious hexavalent chromium which is a cancerogenic substance so as to solve the ecological problems. Further, although the chromate film as formed by the chromate solution is of a large layer thickness and has excellent corrosion resistance, water content is expelled by heat applied when the resin layer is formed to make the coating film brittle, which leads to fine cracks by plastic deformation caused by subsequent bending fabrication or flare fabrication at tube ends, to bring about a problem of reducing the inherent anti-rust performance.
It is, accordingly, an object of the present invention to provide a corrosion resistant resin coating structure of excellent corrosion resistance, weather resistance and chemical resistance, having mechanical strength and tough and having a high heat resistant temperature.
The present inventor has made various studies for solving the foregoing problems and attaining the above-mentioned object and, as a result, has accomplished the present invention based on the finding that the above-mentioned object can be attained by forming a chromate film by using a chromate solution containing only trivalent chromium ions.
That is, the present invention provides a corrosion resistant resin coating structure of a metal tube comprising a metal tube, a zinc or zinc/nickel plating layer formed on an outer circumferential surface of the metal tube, a chromate film comprising a trivalent chromium compound formed on the plating layer and a kind of resin layer selected from the group consisting of polyvinyl fluoride, polyvinylidene fluoride, polypropylene, polyethylene and polyamide resins formed on the chromate film. The present invention further provides a corrosion resistant resin coating structure comprising a metal tube, a zinc or zinc/nickel plating layer formed on an outer circumferential surface of the thin-walled fine diameter metal tube, a chromate film comprising a trivalent chromium compound formed on the plating layer, a primer selected from the group consisting of epoxy resin formed on the chromate film, polyamide resin, silane coupling agent and a kind of resin layer selected from the group consisting of polyvinyl fluoride formed through the primer, polyvinylidene fluoride, polypropylene, polyethylene and polyamide resins.